"Forecasters gave ample warning of torrential rain on Saturday," said local councillor Jason Humphreys.

"But once again flood management agencies failed in their duty as they seem to be completely unaware of the dangers to people and property."

But NRW said event organisers were made fully aware of flood warnings, and the final decision to use the Porthmadog site was taken by the Festival No.6 teams.

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Media captionBethan Williams, from Llangefni was one of those who was forced to stay at a leisure centre

"It's an area that does flood quite regularly - it's a flood plain," said Tim Jones, from the agency.

"We put out a flood warning that afternoon (Saturday) and we spoke to the organisers when those went out.

"It was their decision to make - we had teams out all through Sunday night and Saturday. We've done everything we could do in the situation."

A statement on the festival's website warned to expect long delays after the car park reopened following an overnight closure on safety advice.

On Monday afternoon, they posted a message on their social media accounts stating 90% of cars had now left.

"We are working with 18 tractors to ensure that the remaining 10% are exited as swiftly as possible while taking care to safeguard people's wellbeing and property," added organisers.

But some festival goers at the three-day event in the Italianate village of Portmeirion had already abandoned their vehicles and made their own way home.

Image caption
Some cars were stuck in pools of water on Sunday

One reveller, called Tim, was having his car towed to London after it was damaged while being dragged out of the mud.

"No-one really knew what was going on," he said. "We got down there to find it completely covered - waist-deep water - people on top of cars waiting for tractors.

"People have been there for 12 hours with kids. It was quite scary."

Red Cross help

"It was who had the most money - almost like Mad Max - people bribing £200 to get people out."

NRW said the situation at the site would have been much worse without the work of teams to manually open tidal flood gates to help drain the area.

Image caption
Around 200 people took refuge in Porthmadog's Glaslyn leisure centre on Sunday night

The Red Cross was involved in helping Gwynedd council provide help to people who stayed overnight at Glaslyn leisure centre in Porthmadog, while a Tesco store in the town opened to offer supplies.

"Six volunteers are currently working from the rest centre in Porthmadog and another six volunteers are on hand to set up a second rest centre at a different site if needed," said Red Cross service manager David Hallows.

"A further 20 volunteers are on standby to provide support if the situation extends into a second night."

Responding to the call for an inquiry into the weekend flooding, a spokesman for Gwynedd council said the focus was on helping those still stranded.

"As with any other festival or major event, we will be meeting with the Festival Number 6 organisers and other partners in the near future to look back and reflect on the arrangements," a council official added.